This invention relates to the catalytic, vapor phase oxidation of n-butane to maleic anhydride. More particularly, the invention relates to an improved process for production of maleic anhydride wherein (1) a n-butane rich oxidation feed is oxidized to maleic anhydride in a heat transfer medium-cooled, tubular reaction zone, containing a fixed bed of catalyst graded in terms of reactivity, at relatively low per pass conversions of n-butane, (2) reactor effluent is withdrawn from the reaction zone and a major portion of maleic anhydride is separated therefrom, and (3) a major portion of the effluent remaining after separation is recycled to the reaction zone.
Catalytic, vapor phase oxidation of n-butane to maleic anhydride in heat transfer medium-cooled, tubular reaction zones is well known. Typically, a gaseous feed comprising molecular oxygen, n-butane and ballast gas is passed over a fixed bed of oxidation catalyst in one or more reaction tubes at temperatures of about 300.degree. to about 650.degree. C. and pressures of about 10 to about 75 psia. The oxidation reaction is highly exothermic and, in order to maintain the desired reaction zone temperature, heat transfer medium such as an oil or molten salt is circulated around the reaction tube or tubes. Typically, temperature of the heat transfer medium is adjusted to provide adequate cooling at the hottest point of the reaction zone. Effluent, typically comprising maleic anhydride, by-product oxygenated hydrocarbons, inert gases and unreacted n-butane and oxygen, is withdrawn from the reaction zone and maleic anhydride is substantially separated therefrom.
At present, known commercial processes for producing maleic anhydride from n-butane can be characterized as once through, air oxidation processes in that air is used as the source of molecular oxygen and, owing to the nitrogen content of air, levels of nitrogen in the reaction zone effluent build up to such an extent that recycle of effluent, with or without separation of nitrogen from unreacted n-butane, is economically impractical. In view of the flammability of mixtures of n-butane and air, concentrations of n-butane in a once through, air oxidation feed typically are limited to about 1.8 mole%, and even this is slightly within the flammable region. As a result of such limitations on feed n-butane, the amount of maleic anhydride that can be produced per unit of reaction zone capacity is limited. Further, due to the impracticality of recycling unreacted n-butane, the same typically is discarded such that the amount of maleic anhydride produced per amount of n-butane consumed is lower than would be desirable.
In an attempt to improve productivity and n-butane consumption, it has been proposed to employ oxidation feeds containing higher concentrations of n-butane than typically are used in the above-described once through, air oxidation processes and/or to recycle reaction zone effluent. For example, U.S. Pat. No. 3,899,516 (Dickason) discloses that space time yields and catalyst selectivity to maleic anhydride can be improved through the use of a feed containing n-butane and substantially pure (at least 95%) molecular oxygen in a molar ratio of at least 1:4, i.e., at least 20 mole% n-butane and less than 80 mole% oxygen in the feed. Commonly assigned U.S. Pat. No. 3,904,652 (Frank) discloses the use of feeds containing greater than 1.7 mole% n-butane, 3-13 mole% oxygen, and 70-95 mole% inert gas, preferably nitrogen, in conjunction with 30 to 70% per pass conversions of n-butane and recycle of reactor effluent after separation of maleic anhydride in order to attain improved selectivity to maleic anhydride and ultimate conversions of n-butane. Similarly, U.S. Pat. No. 4,044,027 (Anderson et al.), which is directed primarily to improving product quality and yield by rapid cooling of reactor effluent so as to avoid decomposition of maleic anhydride and formation of color bodies, discloses the use of feeds containing at least 1.5 mole% n-butane and less than 20 mole% oxygen in conjunction with less than 50% per pass conversion and recycle of a portion of the reactor effluent after separation of maleic anhydride and at least some unreacted n-butane therefrom. W. German Offen. No. 2,544,972 (Hoechst) discloses a process for producing maleic anhydride from mixed butane isomers wherein a butane and air feed is oxidized, followed by removal of effluent from the reaction zone, separation of maleic anhydride from the effluent, recycle of 75 to 98% of the effluent remaining after separation to the reaction zone with addition of make-up air and butanes, adsorption of butanes from the remaining 2 to 25% of the effluent on active carbon, desorption of butanes with fresh air and charging of the butane-laden air to the reaction zone.
Despite the improvements reported in the above-described patents, the same are not entirely satisfactory from the standpoint of operation of the heat transfer medium-cooled reaction zone. As noted hereinabove, temperature of the heat transfer medium is determined on the basis of the temperature at the hottest point of the reaction zone. That point, also referred to as "hot spot," is located at the point at which the oxidation rate is maximum and the reaction is most exothermic. Given the positive dependency of reaction rates on reactant concentrations, reaction zone hot spot typically is located near the feed end of the zone because that is where reactant concentrations are highest. In the above-described butane-rich processes, feed end reaction rates are even greater than in typical air oxidation processes due to increased n-butane concentrations. Above a certain concentration of n-butane, heat of reaction cannot be removed due to poor heat transfer in the reaction zone. As a result, catalyst damage and/or thermal runaway of the oxidation reaction can occur. Even if the n-butane concentration is not so high as to cause a thermal runaway, attempts to remove heat of reaction from the reaction zone feed end by adjustment of heat transfer medium temperatures can lead to problems downstream from the feed end. Thus, reactants are consumed as they progress from the feed end of the reaction zone to the exit end with the result that reactant concentrations, and accordingly reaction rates and the amount of heat liberated, decrease. While the heat transfer medium provides adequate cooling in the vicinity of the reaction zone hot spot, the decreased amounts of heat liberated downstream from the feed end typically are insufficient to compensate for the degree of cooling provided near the feed end. Thus, downstream from the feed end of the reaction zone, reaction rates are limited not only by decreased reactant concentrations, but also by excessive cooling. As a result, productivity suffers.
It has been proposed to improve temperature control and operation in various processes involving exothermic reactions by employing a reaction zone in which catalyst activity varies along the length of the zone and such proposals may be of interest with respect to the present invention. Thus, British Pat. No. 721,412 (Chem Patents) discloses oxidation of olefins, and particularly ethylene, in reaction tubes immersed in cooling medium and packed with supported silver catalyst in such a manner that activity increases from the inlet to the outlet of the tubes. Dilution of effluent remaining after removal of product followed by recycle of the diluted effluent also is disclosed. According to the patentee, the use of a graded catalyst results in increased productivity and allows for sustained operation without hot spot formation; however, there is no suggestion that grading of the catalyst bed can be employed to allow for operation on the hydrocarbon rich side of the flammability zone, nor is it disclosed to employ graded catalyst as a means for improving productivity. Further, results in the silver-catalyzed oxidation of ethylene cannot be readily translated to processes for oxidation of n-butane to maleic anhydride.
G. F. Froment, Industrial and Engineering Chemistry, 59 (2), pp. 18-27 (1967) proposes a two-dimensional model for hydrocarbon oxidations in fixed bed, tubular reactors and suggests that hot spot can be eliminated and average reaction zone temperature can be increased by appropriate dilution of catalyst with inert packing in the early stages the catalyst bed.
Caldwell and Calderbank, British Chemical Engineering, 14 (9) pp. 1199-1201 (1969) discuss the aforesaid problems of temperature control in tubular reactors and propose dilution of catalyst in the vicinity of the reaction zone hot spot as a means for attaining increased conversions without making the reaction zone more sensitive to variations in operating conditions. The authors also postulate an equation for calculating the catalyst dilution, as a function of temperature and conversion, required to achieve desired temperature gradients.
Calderbank, Caldwell and Ross, "Proceedings of the Fourth European Symposium on Chemical Reaction Engineering," Pergamon Press, Oxford, pp 93-106 (1971) present one- and three-dimensional models of exothermic catalytic reactions carried out in fixed-bed reactors containing graded catalyst.
Calderbank and Caldwell, Chemical Reaction Engineering, Advances in Chemistry Series, 109 ACS, Washington, D.C., pp 38-43 (1972) discuss oxidation of o-xylene to phthalic anhydride in tubular reactors containing graded catalyst.
Smith and Carberry, The Canadian Journal of Chemical Engineering, 53, pp. 347-349 (1975), although silent with respect to grading of catalysts, disclose the use of partially impregnated catalysts, primarily in conjunction with the oxidation of naphthalene to phthalic anhydride, and conclude that improved yields, higher average reaction zone temperatures and decreased catalyst consumption can result.
Despite the teachings of the aforesaid British patent and the theory and mathematical relationships presented in the literature references, the same do not disclose or suggest the use of a graded catalyst bed to permit smooth operation on the hydrocarbon rich side of the flammability zone nor as a means for permitting economically practical recycle in the production of maleic anhydride from n-butane. The references also fail to disclose actual reactivity gradients useful in oxidation of a given feed to a given product under given conditions. Further, the specific processes addressed by the references, e.g. oxidation of ethylene to ethylene oxide, o-xylene to phthalic anhydride and naphthalene to phthalic anhydride, are sufficiently different from oxidation of n-butane to maleic anhydride as to preclude direct application of the references' teachings and theory to n-butane oxidation. Thus, for example, for a given volume of gas, operation at hydrocarbon concentrations at the lower explosive limits results in evolution of twice as much heat in the case of n-butane oxidation as in the case of oxidation of o-xylene to phthalic anhydride. Accordingly, heat removal requirements in the former are more severe.
From the foregoing, it can be appreciated that it would be desirable to provide a process for production of maleic anhydride wherein the advantages of a n-butane-rich oxidation feed can be better exploited. It is an object of this invention to provide such a process. A further object of the invention is to provide for increased productivity per unit of reaction zone capacity in the production of maleic anhydride from n-butane. A further object of the invention is to provide for improved utilization of n-butane. Another object of the invention is to provide a process wherein recycle of reaction zone effluent is made economically practical. A still further object of the invention is to provide a process that can be operated safely on the hydrocarbon-rich side of the flammability envelope. A further object is to provide a process that can be implemented in conventional once through air oxidation facilities without loss of substantial capacity. Other objects of the invention will be apparent to persons skilled in the art from the following description and the appended claims.
We have now found that the objects of this invention can be attained by contacting a n-butane rich oxidation feed with oxidation catalyst at relatively low per pass conversions of n-butane in a heat transfer-medium-cooled tubular reaction zone in which the catalyst is graded in terms of reactivity, with recycle of a major portion of maleic anhydride-free reaction zone effluent to the reaction zone. Advantageously, the use of a n-butane rich feed in conjunction with graded catalyst results in improved productivity of maleic anhydride as compared to typical air oxidation processes and n-butane rich processes using ungraded catalyst. Further, grading of the catalyst bed results in a more uniform temperature profile over the effective length of the reaction zone such that reaction zone capacity is better utilized. In addition, operation at relatively low per pass conversions can give improved selectivities to maleic anhydride, and this in combination with recycle of unreacted n-butane can result in improved yields of maleic anhydride. Ultimate conversions of n-butane also are improved. As compared to conventional, once through air oxidation processes, operation according to this invention can give up to twice the throughput for a given reaction zone capacity, with decreased catalyst consumption due to grading of the catalyst bed.